In SE-B-469 466 a two step electro filter having an ionization section is described, said section being downstream followed by a so called precipitator. The electrode elements of the precipitator according to the mentioned patent application constitute of planar plates that comprise high resistant non-metallic material, said material also being designed as antistatic (so called dissipative material). By such a material an essential improvement of the separation ability is achieved compared to the traditional design, i.e. electrode elements of the precipitator comprising metallic material, and the voltage between the electrode elements can reach a higher level than in connection with traditional electrode elements manufactured from material having low resistivity. Further a new design for the ionization chamber is disclosed, said design being very simple and very efficient in terms of particle charging and with an exceptionally low emission of ozone.
In SE 9303894-1 is described a further development of the design of a precipitator according to the patent application mentioned above. By screening of the edge sections of the electrode elements of the precipitator possibilities are achieved, according to this patent application, to further increase the voltage between said elements and thus an increased separation ability.
In spite of an improved performance and new design of the electro filter cassette, inter alia from a cellulose based material and very simplified design of the ionization chamber, said inventions have not initiated an increased use of air purification devices and also not in other applications as for instance filters in ventilation ducts, coupe filters, filter for cooking fumes, a so called cooker hood, or in more industrial application areas, although the need of fresh air no longer might be questioned and although the electro filter technique in many aspects is much better than the traditional filter technique comprising so called barrier filters.
The reason for this very restricted use of the electro filter technique might be that systems for air purification also must fulfil other essential demands, apart from the air purification efficiency, to become accepted within wider circles of users.
These demands vary essentially depending on application area but they are also very different within the same application area for different users and different environments having different pollution load etc.
To exemplify certain demands that must be set up within at least one area of use, for instance separate air purifiers as a complement to existing ventilation, one can mention low initial cost/running charges per m.sup.3 purified air, low noise level, flexible location possibilities and adaption to different interiors, a simple way to operate the set-up, low cost for replacement cassettes, the way to handle used filter cassetes adapted both to the environment but also to the status of the user, for instance a healthy person or a person allergic to dust/an asthmatic person, long life of the devices etc.